Rock Cycle - Rocks (5B)
Pre Lab 

  • Exploring the creation of caves.
  • Comparing stalactites and stalagmites.
  • calcite
  • dissolve
  • stalactite
  • stalagmite
  • two glass jars
  • warm water
  • baking soda
  • wool thread or mop tread

Students make a stalactite and stalagmite.

Ohio Caverns


Many areas of the Earth’s surface were once under the oceans. Large coral reefs covered many of these areas. For example, Texas and New Mexico were living coral reefs at one time, larger than the present Great Barrier Reefs in Australia. These great reefs became rock as the sea retreated from the land. This rock consists of the remains of shelled animals, which are composed mainly of calcium carbonate (calcite). This type of calcium carbonate rock is called limestone. Limestone will dissolve if acid is put on it. Geologists usually use a 10% HCl solution for this "fizz" test.

Groundwater percolates through soil and rock within the earth. Groundwater is naturally acidic. When groundwater percolates through limestone, it dissolves it. This is how caves form.

The dissolved limestone is transported in solution by groundwater. Stalactites and stalagmites form as groundwater drips from the ceiling of a cave. Some of the water evaporates, and new limestone precipitates from the remaining saturated solution. This builds hanging pillars of limestone called stalactites, and stalagmites, limestone pillars that rise from the floor of the cave. You can remember that stalactites hang from the top, by remembering the "t" in "tite," stands for the "t" in "top." Limestone formed by this type of inorganic precipitation is called travertine.

  1. Before lab, gather the materials needed for each student group. You may wish to run the experiment once before class to get a feel for how quickly the "stalactites" form.
  2. Illustrate how stalactites form with the following experiment. Make a solution of warm to hot water and dissolve baking soda in it until it forms a saturated solution. It is easier to boil water and stir in as much baking soda as will dissolve.
  3. Fill two small containers half full with the baking soda solution. Place the containers about 20 cm apart, and put a small saucer or lid between them. Saturate the mop thread with the solution. Put an end of the mop thread into each container. Suspend the thread between the two containers. Crystals of baking soda (or tiny stalactites) will slowly form in the saddle area of the thread. Use the picture below as a guide.

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